I. Field of the Invention
The present invention relates generally to the operation of cardiac rhythm management systems and, more particularly, to the optimization of cardiac resynchronization therapy based on templates produced pursuant to data exhibiting trending of selected conduction times within the heart.
II. Related Art
Early cardiac pacemakers were used primarily to pace the heart when a normal conduction path from the sino-atrial (SA) node of the heart to the atrial-ventricular (AV) node or from the AV node to the ventricles was interrupted. In accordance with these events, the pacemaker was called upon to deliver ventricular stimulating pulses to maintain a pre-determined heart rate. More recently, pacemaker technology has become greatly advanced and sophisticated. For example, rate adaptive pacing has been used to vary the pre-determined rate in accordance with parameters indicative of patient activity level. In addition, techniques now involve the pacing of multiple chambers and even sequentially pacing multiple sites in the same chamber.
Thus, it is well recognized that patients having cardiac disorders may receive benefits from cardiac pacing. One such well recognized disorder is congestive heart failure (CHF). Generally, CHF refers to a cardiovascular condition in which abnormal circulatory congestion exists as a result of an inability of a heart to maintain the necessary circulatory flow rate. Circulatory congestion is a state in which the heart enlarges to compensate but the stroke volume decreases. Reduced cardiac output can be due to several disorders, including mitral regurgitation which involves a backflow or leakage of blood from the left ventricle to the left atrium and intrinsic ventricular conduction disorder which involves an asynchronous contraction of the ventricular muscle cells. These are the two common abnormalities among CHF patients and normally occur together to a greater or lesser degree in such conditions.
One recognized and accepted indication of hemodynamic performance is reflected in the patient's pulse pressure (PP) which is defined as the difference between the systolic aortic pressure and the diastolic aortic pressure. While PP can be directly used to optimize the pacing parameters in applying CHF therapy, this would require the use of a suitably positioned pressure sensor. This technique is preferably avoided because implementation of such systems require complicated measurement and none have been adapted to provide automatic optimization of cardiac performance parameters based on the measurements.
It has further been recognized that the timing of pacing may be used to provide improvements in aortic pulse pressure and that this may be achieved by adjusting the atrio-ventricular (AV) delay time or interval, which is the time interval after a sensed P-wave, to delivery of a ventricular pacing pulse to achieve a desired cardiac parameter optimization. As shown in the graphical representation of AV delay in FIG. 3, the optimization of AV delay has a profound effect on observed pulse pressure. This may be crucial to maximize the benefit of pacing, particularly for CHF patients or those with bradycardia indication. As can be seen from that figure, a longer AV delay (above 225 ms) provides little or no hemodynamic benefit; whereas an optimized AV delay of about 75 ms, for instance, can increase or boost pulse pressure by as much as 25%.
It has also been shown that optimum AV delay can be predicted from the intrinsic atrio-ventricular conduction time (AVCT) using a mathematical relationship. Such a mathematical relationship is shown and described in U.S. Pat. No. 6,144,880 to Jiang Ding et al and assigned to the same assignee as the present invention. That patent is deemed incorporated herein by reference for any purpose. It is recognized that intrinsic or natural AVCT may be a time measured between the atrial depolarization time (onset of P-wave) and any selected morphological marker of ventricular depolarization such as Q or R or S. This is further illustrated in FIG. 4 in which PQ′ and PR spans are indicated.
In view of the above, a great advantage could be gained by providing a more accurate approach to the on-going determination of conduction time between two points within the heart such as AVCT and others. This could beneficially provide a more accurate and continuously updated basis for optimizing delays between sites including the AV delay whether its use suggests to the user a fixed delay value or a dynamic value which changes as a function of cardiac cycle length, activity or minute ventilation levels, or other selected parameter of interest.